1. Field of the Invention
The present invention relates to an image forming apparatus for effecting image formation on a recording member, such as a copier.
2. Description of the Prior Art
There are already known copiers with variable image magnification for enlarging or reducing the original image. In such copiers the distribution of the light intensity on the photosensitive drum has to be uniform both in the one-to-one copying mode and in the copying mode with modified image magnification, and such uniform light distribution has been conventionally achieved by the use of a light distribution correcting plate, such as a slit, positioned in the vicinity of the photosensitive drum. An example of such correction for light distribution in the actual size copying, reduction copying and enlarged copying is shown in FIGS. 1A, 2B and 1C. The original illuminating system is so constructed as to compensate the so-called cos.sup.4 .theta. rule of a lens 50 in the real-size copying mode. Thus the illuminating system has a distribution of illumination intensity on the original document as represented by a curve 100 to obtain a uniform intensity distribution on the photosensitive drum as represented by 200 after passing the lens 50. However, in case the image magnification is modified, the distribution of the illumination intensity becomes uneven as represented by 201 or 202, respectively in case of size reduction or size enlargement, since the angle of viewing the original document from the lens changes in such cases. Such unevenness has been corrected as represented by 201' or 202' by the insertion of a slit 301 or 302, respectively in case of size reduction or size enlargement, of which forms are respectively shown in FIGS. 2A and 2B.
The use of such slits in the modified size copying mode enables to obtain a uniform light intensity distribution on the photosensitive drum, but at the same time results in a loss of light amounting even to about 30%, because of partial shielding of the light by said slit. Such loss of light is compensated by employing a lower process speed in such modified size copying mode than, for example equal to 0.7 times of, the process speed in the real-size copying mode, and the amount of light at such modified size copying mode is rendered adjustable, for example with a slit, to 0.7 times of that in the real-size copying mode. Also the use of such lower process speed in the modified size copying mode prevents the vibration of image which may be caused, in the reduction copying, by a high scanning speed (=process speed/image reduction rate).
Also there is already known a control process for chargers, developing device etc. in response to the detection of surface state, for example surface potential of the photosensitive member in order to achieve stable image reproduction. FIG. 3 shows an example of such control process, wherein a three-layered photosensitive drum 500, having an insulating layer, a photoconductive layer and a conductive layer in this order from the external periphery, is surrounded, in the order same as the direction of rotation thereof, by a primary charger 501 for uniformly charging said drum, a secondary charger 502 for charge elimination and a whole-surface exposure lamp 503. An original document placed on a carriage is illuminated by a light source 504 such as a halogen lamp, and the reflected light is focused through a lens 506 onto the photosensitive drum 500 at a position the charge on said drum according to the amount of exposure to the original image, thereby forming an electrostatic latent image on the drum corresponding to the original image. The electrostatic latent image thus formed is entirely exposed to the light from the whole-surface exposure lamp 503 to obtain a latent image with improved gradation. Thereafter the latent image moves to a developing device and is developed with toner by a developing roller 514 to which a bias voltage is supplied. A blank exposure lamp 507 constantly illuminates the photosensitive drum when the original exposure lamp 504 is not lighted while the chargers are in operation, in order to prevent the toner deposition in the non-image area. In the vicinity of the photosensitive drum 500 and between the developing device and the whole surface exposure lamp 503, there is provided a surface potential sensor 505 for measuring the surface potential of the drum. The output signal of said sensor is amplified and converted into digital signals in a surface potential measuring circuit 508, and is then supplied to a potential control circuit 513 composed for example of a microcomputer for effecting data processing according to the measured surface potential. The results of said processing are converted into analog signals and are supplied to high-voltage generating circuits 509, 510, a developing bias circuit 511 and an exposure control circuit 512 for respectively controlling the voltages supplied to the primary and secondary chargers, the developing bias voltage and the voltage to the halogen lamp. The control of the image forming conditions in the above-described apparatus is achieved in the following manner.
After the start of power supply, the drum is subjected to a pre-rotation step for stabilizing the performance of the photosensitive member. Then reference currents I.sub.po, I.sub.so are respectively supplied to the primary and secondary chargers 501, 502, and the surface potential sensor 505 measures the dark potential V.sub.D after the entire illumination with the
After the start of power supply, the drum is subjected to a pre-rotation step for stabilizing the performance of the photosensitive member. Then reference currents I.sub.po, I.sub.so are respectively supplied to the primary and secondary chargers 501, 502, and the surface potential sensor 505 measures the dark potential V.sub.D after the entire illumination with the whole surface exposure lamp 503, and the light potential V.sub.SL after the illumination with the blank exposure lamp 507 at the highest intensity. Then the primary and secondary currents I.sub.p, I.sub.s are corrected so as to bring the light and dark potentials V.sub.SL V.sub.D closer to the target values, and such correcting cycle is repeated for example four times.
Then the original exposure lamp 504 is lighted with a reference voltage V.sub.HO, and the surface potential sensor 505 measures the potential V.sub.L of the latent image formed on the photosensitive drum corresponding to a standard white plate. Then the lamp voltage V.sub.H is corrected so as to bring said potential V.sub.L closer to zero, and this cycle is repeated for example three times. The developing bias voltage is obtained by adding a determined voltage to said potential V.sub.L. The above-described control allows to bring the photosensitive characteristic, for example represented by a full-lined curve in FIG. 4, to an ideal characteristic represented by a broken-lined curve. The succeeding copying cycle is conducted with thus corrected primary and secondary charging currents I.sub.p, I.sub.s and lighting voltage V.sub.H.
Such control is conducted, also in the modified size copying mode, with the optical path and the process speed for the real-size copying mode to determine I.sub.p, I.sub.s and V.sub.H in the aforementioned manner, and the primary and secondary charging currents in the modified size copying mode are obtained by multiplying for example 0.7 with said values I.sub.p, I.sub.s if the process speed in the modified size copying mode is 0.7 times of that in the real-size copying mode.
Such control process however requires a long time and involves the complexity of maintaining required mechanical precision, since, in the modified size copying mode, the optical system is at first returned to the position for the real-size copying mode and is then brought to the position for the modified size copying mode after the potential control in the aforementioned manner.
Also in such process, the image density and the gradation of intermediate image tone in the modified size copying mode become different from those in the real-size copying mode, since the reciprocity does not stand in strict sense.
In addition, the number of corrections for the charging currents and for the lighting voltage is determined in advance, so that the correcting operations have to be repeated wastefully even when the charging current or the lighting voltage is already at the target value or when the correction is no longer possible because of the limitation in the capacity of the power supply.